The purpose of the proposed research program is to obtain detailed understanding of the mechanisms of electrolyte transport by mammalian colon. Particular emphasis is placed on the modification of these processes by humoral and pharmacologic agents. Current studies are being focussed on the mechanism of action of aldosterone on sodium and potassium transport by large intestine in an attempt to define the site(s) of action of the mineralocorticoid on the active Na transport pathway as well as those factors which determine or influence potassium secretion. Additional studies are directed toward an evaluation of the rate-determining steps of the active Na transport path and an analysis of the equivalent electrical circuit model describing Na transport. Finally, the factors influencing active chloride secretion by the large intestine are being evaluated with emphasis on those intracellular mediators of the secretory response. BIBLIOGRAPHIC REFERENCES: Frizzell, R.A. Interactions between cyclic AMP and cell calcium in the stimulation of electrolyte secretion by mammalian colon. In: Hormonal Receptors in Digestive Tract Physiology. S. Bonfils, P. Fromageot and G. Rosselin, Eds. North Holland Publishing Co., Amsterdam. p. 455, 1977. Frizzell, R.A. Relations between electrolyte transport mechanisms of various epithelia and their responses to cyclic AMP. In: Hormonal Receptors in Digestive Tract Physiology. S. Bonfils, P. Fromageot and G. Rosselin, Eds. North Holland Publishing Co., Amsterdam. p. 483, 1977.